Gas saturation of the non-traditional reservoir of the Lower Berezovskaya subformation of the north of the West Siberia, and its relation to the void mineral composition and structure

The oldest Cenomanian gas fields of the north of the West Siberia have a steady tendency of the production decline yet today. The Lower Berezovskaya subformation gas was ignored before, now it attracts attention of the producer companies more often. The collector of this subseries is a complex underexplored non-traditional object promising to support gas production. Its mineral composition and basic petrophysical properties have recently been studied actively, however, the issue of the gas saturation level is still open.Argillous gaizes represent the Lower Berezovskaya subformation reservoir. This work provides the first evaluation of the saturation in this reservoir based on the interpretation of the geophysical studies of wells and encapsulated core analysis. It also reveals and describes the connection between the mineral composition of the gaizes, the structure of the voids, and the gas saturation.
Materials and methods
The DMRP (Density Magnetic Resonance Porosity) method was used to define the level of gas saturation in wells. The argillous gaizes were studied using the Preobrazhensky method and non-stationary helium filtration method. They also used an X-ray diffraction and adsorptive structural analyses. The extraction distillation method was used to study the encapsulated core samples.
Results
Production well logging data analysis allowed allocating two significant objects in the Lower Berezovskaya subformation, which are reservoirs LB1 and LB2.The improved DMRP method allowed calculating the gas saturation rate of the near well bore area. It is established that the gas saturation of the argillous gaizes depends on the void space mineral composition and structure. The reservoir LB1 is less gas saturated than LB2, as it contains low crystalline and amorphous silica modifications (opal christobalite tridymite phase), whereby the void space is mainly represented by the subcapillary porosity filled with molecular water.
Conclusions
According to the capillary mechanism of the accumulation, large capillary dimension voids alone contain gas (macroporosity classified by IUPAC). Due to this fact, the most important zone of interest production wise in the Lower Berezovskaya subformation is reservoir LB2 with its void space of more than 50% of macroporosity. Subsequently, using high-resolution tomography and microscopy Digital Core will allow describing the mechanism of the formation of void space structure of the gaizes depending on the morphology of the silica. Considering the view of the gas saturation developed in this article, this information will allow developing a correct electrical model of the non-traditional reservoir of the Lower Berezovskaya subformation. This, in its turn, will be a basis for determining the true gas saturation according to results of the borehole electrical measurements.